Dunning Letters: What They Are, How They Work, and When They Stop Working

Most dunning letters arrive too late to matter.

By the time an invoice hits 90 days past due and your credit team sends a formal collections notice, the problem started 60 days earlier when the first invoice aged without anyone noticing. A well-run dunning process is valuable. A well-run monitoring process is more valuable. One prevents the fire; the other hands you a bucket after the building is already burning.

With that context established, here's how dunning letters work and how to run them effectively.

What a Dunning Letter Is

A dunning letter is a written notice sent to a B2B customer who has not paid an invoice by its due date. The term comes from the 17th-century English word "dun," meaning to press a debtor for payment. The modern dunning letter serves the same function: structured, documented communication to collect on past-due receivables.

Dunning letters serve multiple purposes beyond collection. They create a paper trail that supports escalation to third-party collections or legal action. They establish that you provided notice. And they give customers who are experiencing financial difficulty a formal channel to respond before the situation escalates.

The Standard Dunning Letter Sequence

Most B2B credit teams run a three-to-four-stage dunning sequence.

Stage 1: First Notice (1-15 Days Past Due)

The first notice is a courtesy reminder. It assumes the non-payment is an oversight, not a dispute or cash flow problem. The tone is professional and neutral. Include the invoice number, invoice date, due date, amount owed, and payment instructions. Do not threaten. Do not assume bad faith. Many customers with slow payment workflows will resolve this at stage one.

Stage 2: Second Notice (15-30 Days Past Due)

The second notice escalates slightly. It references the first notice and confirms that the account remains past due. It explicitly states the amount owed and any applicable late fees under your credit terms. It asks for payment within a specific number of days, seven to ten is standard, and provides a contact for payment arrangements if the customer has a cash flow issue.

At this stage, the credit team should also flag this account internally. A customer who doesn't respond to two payment notices may be experiencing financial distress. That's a signal, not just an AR problem.

Stage 3: Final Notice (30-60 Days Past Due)

The final notice is a formal demand. It states clearly that if payment is not received by a specific date, the account will be referred to a collection agency or attorney. Use direct language. Specify the exact amount owed including fees. Do not leave the outcome ambiguous.

This is also the point to review the account's credit status. Customers at 60 days past due with no response to three formal notices should not be receiving new shipments on terms. If your credit monitoring is working, this review should have happened two weeks ago.

Stage 4: Collections Referral (60+ Days Past Due)

At 60 to 90 days past due with no response to three formal notices, the account moves to third-party collections or legal. The transition letter informs the customer that the debt has been referred and that further communication should go to the collections agency or attorney. Keep this letter factual. Do not use language that could be read as harassment.

Legal Requirements for Dunning Letters

Dunning letters in B2B contexts are governed primarily by common-law contract principles and, in some cases, state-specific commercial debt collection statutes. The Fair Debt Collection Practices Act (FDCPA) applies primarily to consumer debt. But many states have laws that extend similar protections to business debtors, and violations create legal exposure.

The practical rules: do not misrepresent the amount owed, do not threaten legal action you don't intend to take, do not contact parties other than the authorized accounts payable contact without a specific reason, and retain copies of every letter sent with send dates documented. These records matter if a dispute goes to court.

Have your collections attorney review your dunning letter templates before deployment. The cost of a legal review is far less than the cost of an FDCPA-adjacent claim, even in a B2B context.

What Every Dunning Letter Should Include

Every dunning letter, regardless of stage, needs:

• Your company name and contact information
• The customer's company name and billing address
• Invoice number(s) and original invoice dates
• Original due date(s)
• Amount past due, broken out by invoice if multiple
• Any late fees or interest applied under your credit terms
• Total amount owed
• Payment instructions (check, ACH, wire, online portal)
• A specific response deadline
• A contact name, phone, and email for the collections team

What to leave out: passive language that lets customers dismiss the notice as routine, ambiguous language about consequences, and vague deadlines like "at your earliest convenience." Every dunning letter should have a date and a number.

Dunning Letters and Your Credit Policy

Dunning letters don't exist in isolation. They're one step in a credit policy that defines how your team extends credit, monitors accounts, and responds to deterioration. Without a documented credit policy, the dunning process is improvised at best.

A credit policy defines: who qualifies for net terms and at what limits, what payment terms apply, what late fees accrue and when, how disputes are handled, and what triggers escalation to collections or legal. The dunning letter sequence enforces that policy. It's not a substitute for having one.

For a full overview of how trade credit works and how to structure the terms your dunning letters will eventually reference, see our guide on trade credit.

For a step-by-step walkthrough of writing each letter in the sequence, see our post on how to write a dunning letter.

Why Dunning Alone Is Not Enough

A dunning process tells you what already happened. It doesn't tell you what's happening now with customers who are still current on paper but deteriorating financially.

DSO has the same problem. Days sales outstanding measures payment velocity after invoices are due. By the time DSO climbs, the customer is already past due. You're reviewing the crime scene, not preventing the crime.

The credit teams that avoid the situations requiring aggressive dunning are the ones monitoring accounts between payment cycles. They watch for early warning signs: extended payment patterns, deteriorating financial ratios, UCC lien filings, industry distress signals. They catch the customer who's going to miss the 90-day mark while that customer is still paying on time.

Dunning is necessary. It's not sufficient. For a more complete picture of how to monitor B2B customers before they reach the dunning stage, see our guide to B2B credit risk monitoring.